The invention concerns a nuclear magnetic resonance imaging (MRI) configuration with a magnet system for generating a homogeneous magnetic field B0 in a volume under investigation, a radio frequency (RF) system for generating an RF field B1 in the volume under investigation or for detecting an RF field B1 from the volume under investigation, with a gradient system for temporarily superposing defined gradient fields on the magnetic field B0, and an MRI phantom positioned in the volume under investigation, which aids in determining the homogeneity of the magnetic field B0 and/or the RF field B1 and/or the linearity and/or intensity and/or scaling of the gradient fields in the volume under investigation, wherein the NMR phantom comprises a chamber which is disposed in a housing and filled with a liquid, in which a gas bubble forms, wherein the liquid contains nuclei having an NMR relaxation time T1 of between 100 ms and 20 s.
The company Siemens AG distributes such a configuration e.g. under the name “MAGNETOM Trio”
(http://www.pc.rhul.ac.uk/vision/Restricted/Siemens/-system_trio.pdf).
In order to ensure constant quality of MRI recordings, the above-mentioned properties of the magnetic fields (B0 field, B1 field, and gradient fields) must be tested. A substance is required in order to perform these tests, which is disposed in the image plane in a homogeneous or structured fashion. A hollow body (MRI phantom) filled with a suitable liquid is generally used for this purpose.
When filling liquid into a hollow MRI phantom body, a gas bubble generally forms which can produce imaging artefacts. If the gas bubble is e.g. in the recording plane, no signal is detected in the region of the gas bubble. Moreover, the switching of gradients during MRI recording can cause vibrations of the gas bubble and thereby pressure waves within the liquid in the MRI phantom which produces signal variations within the image plane and thereby artefacts. These problems are known and were discussed e.g. in “Quality Assessment of high spatial resolution for MRI”
(http://ric.uthscsa.edu/personalpages/-lancaste/DI2_Projects—2003/QA_MRI.pdf) and “AAPM Summer School 2001”
(http://www.aapm.org/meetings/2001ISS/presentations/clarke_MRI%20-Troubleshooting.pdf). For this reason, MRI phantoms are regularly refilled in order to minimize gas bubble formation.
Despite such measures to avoid formation of gas bubbles during filling of such MRI phantoms, the liquid may nevertheless be pressed through the seal of the filler neck of the MRI phantom (leakage) when temperatures increase or the hollow body may burst. If there is a gas bubble within the MRI phantom, the compressibility of the gas can compensate for pressure fluctuations. For this reason, the presence of gas bubbles is sometimes even desired, in particular, when liquids containing harmful substances are used.
It is the underlying purpose of the present invention to propose an MRI configuration with an MRI phantom, which avoids imaging artefacts despite the presence of a gas bubble.